1. Field of the Invention
This invention relates to an ophthalmologic apparatus including a means for detecting the alignment of a subject's eye with an observation optical system which can observe the eye at various magnifications.
2. Description of the Prior Art
Heretofore, an ophthalmologic apparatus is known for observing and photographing an image of a corneal endothelium. In this apparatus, a subject's eye and an optical system of the apparatus are first located roughly in respective positions by an operator, and then an image of an anterior segment of the eye is displayed on a display by means of an anterior segment observing optical system. The alignment of the eye with the apparatus is carried out in up, down, right, and left directions, while the anterior segment image is observed. After that, illumination light emitted by an illumination light source for observation in a illumination optical system is projected onto a cornea of the eye and, at the same time, a display image is changed from the anterior segment image to an corneal endothelium image by reception of reflection light reflected by the cornea. Finally, the alignment is carried out in forward and backward directions to observe and photograph the endothelium.
The apparatus is designed to photograph the corneal endothelium when the alignment is the forward and backward directions is completed in order to obtain its image in focus on the assumption that the alignment in the up, down, right, and left directions has already been carried out.
In the apparatus, the magnification of the endothelium image is higher than that of the anterior segment image when those images are displayed on the display. In addition, a reference allowable range for the alignment in the up, down, right, and left directions is fixed, thus being independent of those magnifications.
In other words, the reference allowable range for the alignment carried out when the anterior segment image is displayed is the same as that carried out when the corneal endothelium image is displayed, i.e., when its image is photographed.
Therefore, if the reference allowable range is set large, the alignment is quickly carried out when the anterior segment image is displayed. However, when the corneal endothelium is photographed at a high magnification, its resultant image is often out of focus. On the other hand, if the range is set small, the corneal endothelium is clearly photographed. However, due to the small range, much time is consumed to align the eye with the apparatus when the anterior segment image is displayed and to change the displayed image to the corneal endothelium image.